1. Field of the Invention
The present invention relates in general to a process for fabricating, on a single-crystal silicon or germanium substrate, a stack of silicon and/or germanium quantum box planes and to a device comprising a single-crystal silicon or germanium substrate and a stack of quantum box planes, the size of which is variable in each plane but oriented.
2. Description of the Related Art
At the present time, the formation of silicon quantum boxes has been the subject of many studies. These quantum boxes are generally composed of a single-crystal materials of less than 100 nm in size.
Conventionally, quantum boxes are produced in the form of semi-hemispherical dots approximately 20 nm in length at the base and from 2 to 3 nm in height and are made of III-V compounds. III-V compounds have the advantage of exhibiting direct energy bandgaps (and therefore exhibiting a high efficiency in luminescence for example) but they are difficult to treat and integrate.
In contrast, silicon is a well-controlled material from the standpoint of both its treatment and its integration. However, its indirect energy bandgap is a major handicap in the case of applications requiring high luminescence efficiency. This explains the little interest that this material has aroused in applications of the quantum-box type.
The possibility of coupling silicon (Si) with germanium (Ge) or silicon-germanium alloys (SiGe) has rekindled interest in obtaining quantum boxes made of IVxe2x80x94IV material, that is to say mainly Si or Ge dots.
To produce these dots, chemical vapor deposition (CVD) processes are used. Thus, by carefully choosing the deposition parameters, such as the temperature and the pressure and flow rates of precursor gases (typically SiH4 and GEH4, with H2 as the carrier gas), it is possible to deposit Ge dots on a surface of a silicon layers or Si dots on a surface of an SiO2 layer with sizes of quantum boxes. For example, to increase the luminescence efficiency, the quantum box plane thus produced is encapsulated and then a new quantum box plane is deposited on the surface of the encapsulation layer, and so on over several thicknesses.
The production of such quantum boxes in the form of germanium dots is described inter alia in the articles xe2x80x9cDeposition of three-dimensional Ge islands on Si (001) by chemical vapor deposition at atmospheric and reduced pressurexe2x80x9d by T. I. Kamins, E. C. Carr, R. S. Williams and S. J. Rosner, J. Appl. Phys. Si (1), Jan. 1, 1997 and xe2x80x9cGermanium quantum dotsxe2x80x9d embedded in silicon: Quantitative study of self-alignment and coarseningxe2x80x9d by O. Kienzle, F. Ernst, M. Rxc3xchle, O. G. Schmidt and K. Eberl, Appl. Phys. Lett. Vol. 74, No 2, Jan. 11, 1999.
Semi-hemispherical quantum boxes have several drawbacks.
The quantum boxes obtained are made of single-crystal material but do not all have the same crystal orientation and may even exhibit microtwins.
The size distribution of the boxes is not well centered and the spacing between the boxes is not completely regular (it depends on the chemical vapor deposition nucleation conditions).
During encapsulation of a plane of quantum boxes, there may be interdiffusion of species from the encapsulation material into the quantum box material.
For example, silicon deposited by CVD at a temperature typically greater than 550xc2x0 C. in order to encapsulate the Ge quantum boxes in a plane, will tend to diffuse into the germanium of the boxes.
The formation of quantum boxes by chemical vapor deposition, or another deposition technique such as molecular beam epitaxy (MBE), is essentially a problem of nucleation. Nucleation phenomena are often exponential in nature, requiring control of the duration. Nucleation is also highly dependent on the nature and the state of the receiving surfaces and the latency time is often a limiting factor in obtaining reproducible results.
Therefore, a process for producing a stack of silicon and/or germanium or SiGe alloy quantum box planes, which remedies the above drawbacks, may be desired.
In particular, a process may include fabricating a stack of silicon and/or germanium or SiGe alloy quantum boxes that makes it possible to obtain a succession of quantum box planes with well-defined interfaces, the quantum boxes being virtually the same size, single-crystal, and oriented.
In addition, a device may include a stack of silicon and/or germanium or SiGe alloy quantum box planes, the quaritum boxes being virtually the same size, single-crystal, and oriented.
In an embodiment, a process for fabricating a device having, on a single-crystal silicon or germanium substrate, a stack of quantum box planes, includes:
the formation, on the substrate, of a stack of successive, alternately Si/Ge, Si/SiGe, or Si/SiGe/Ge single-crystal layers in the case of a germanium substrate and conversely in the case of a silicon substrate; and
the electrochemical treatment of the stack of single-crystal layers in order to make the layers porous and to form therein residual crystallites constituting quantum boxes.
In another embodiment, after electrochemical treatment, the stack of porous layers is""subjected to a passivation treatment by electrochemical oxidation. The electrochemical oxidation converts the porous Si layers into insulating SiO2 layers so that a structure is obtained in which the Ge or SiGe alloy layers form planes of quantum boxes between insulating SiO2 layers.
In addition, a device may include a single-crystal silicon or germanium substrate and a stack of quantum box planes, which consists of a stack of porous, Si/Ge, Si/SiGe, or Si/SiGe/Ge successive single-crystal layers in the case of a germanium substrate and conversely in the case of a silicon substrate.
In some embodiments, a device includes a single-crystal silicon or germanium substrate and at least one quantum box plane consisting of a porous germanium or SiGe alloy layer between two SiO2 layers.
In one embodiment, the layers of the stack are Si and Ge layers for maximum chemical, electrochemical, and optical effects.